Level shifters are used in interfaces between circuitry, that sits in different voltage domains. For example, in an integrated circuit, there can be signals between digital circuitry (supplied by 1.0V) and analog circuitry (supplied by 3.3V), as well as input/output systems that typically run at a voltage that is different from the core digital circuitry. Level shifters can be used for these circuit interfaces.
Level shifters are also used within some analog designs, such as power circuitry. In high-voltage applications, such as Class D amplifiers and DC/DC converters, it is often the case that a power stage requires gate drive signals that must be less than the voltage supply, such as due to process limitations. For example, in a 5 V power stage designed in a process that has a 3.3 V oxide, any junction across a single FET must be limited to a voltage level that will not cause the 3.3 V oxide layer to experience a voltage breakdown.